As well known in the art, bonded magnets have many advantages such as light weight, good dimensional accuracy, facilitated moldability into even a complicated shape as well as good mass-production capability as compared to sintered magnets, and, therefore, have been extensively used in various applications such as toys, office equipments, audio equipments and motors.
As the magnetic particles usable in the bonded magnets, there are known rare earth element magnet particles such as typically Nd—Fe—B-based alloy particles, or ferrite particles. The rare earth element magnet particles have high magnetic properties, but are expensive, resulting in limited applications thereof. On the other hand, the ferrite particles are deteriorated in magnetic properties as compared to the rare earth element magnet particles, but are inexpensive and chemically stable and, therefore, have been used in more extensive applications.
The bonded magnets have been usually produced by kneading a rubber or a plastic material with magnetic particles and then molding the resultant kneaded material in a magnetic field or by using a mechanical means.
In recent years, with the increase in performance of various materials or equipments including an enhanced reliability thereof, there is also a demand for enhancing a performance of the bonded magnets including enhancement in strength and magnetic properties thereof.
More specifically, it has been demanded that a molded product of the bonded magnet obtained by an injection molding method, etc., exhibits a mechanical strength capable of withstanding severe conditions when used in various applications.
For example, in copying machines, printers or the like using a magnet roll, since the magnet roll is used at a high speed in these equipments with a long service life which are improved in light weight, thin thickness and reduction in size, it has been strongly demanded that the magnet roll exhibits a high mechanical strength. In addition, as to the magnetic properties of the magnet roll, it has been demanded to achieve, in particular, not only increase in a surface magnetic force of the magnet roll, but also uniformity in the surface magnetic force as important technical factors for obtaining clear images in the copying machines or printers.
For this reason, ferrite particles used in the bonded magnets as well as resin compositions for the bonded magnets which are composed of the ferrite particles and an organic binder are also required to satisfy the above requirements.
Conventionally, there have been made various improvements in the ferrite particles for bonded magnet and the resin compositions for bonded magnet composed of the ferrite particles and the organic binder. For example, there are known the method of producing ferrite particles by using an alkali metal compound or an alkali earth metal compound as a flux (Japanese Patent Application Laid-open (KOKAI) No. 55-145303 (1980)); the method of controlling a particle size distribution of ferrite particles (Japanese Patent Application Laid-open (KOKAI) No. 3-218606 (1991)); the method of mixing two or more kinds of ferrite particles which are different in particle diameter from each other (Japanese Patent Application Laid-open (KOKAI) No. 4-224116 (1992)); and the method of applying an impact force to ferrite particles in a high-speed flow thereof (Japanese Patent Application Laid-open (KOKAI) No. 8-037106 (1996)).
At the present time, the ferrite particles for bonded magnet and/or the resin compositions for bonded magnet which are capable of satisfying the above requirements have been strongly required. However, the ferrite particles and/or resin compositions capable of satisfying the requirements to a sufficient extent have not been obtained until now.
That is, the bonded magnet molded products produced by using the ferrite particles or resin compositions for bonded magnet as described in the above Patent Documents have failed to exhibit all of high magnetic force, uniformity in magnetic force, excellent magnetic strength.